Polyethylene blend



Feb. 9, 1960 M. M. SAFFORD srm. 2,924,559

POLYETHYLENE BLEND Filed larch 27, 1956 [n ven'ors: Moyer- M Safiord, Robert L. Myers, by W 77zeir- Attorney United States Patent POLYETHYLENE BLEND Moyer M. Saiford, Schenectady, and Robert L. Myers, Ballston Lake, N.Y., assignors to General Electric Company, a corporation of New York Application March 27, 1956, Serial No. 574,334

8 Claims. (Cl. 204--154) This invention relates to curable compositions comprising blends of polyethylene and polymerized 1,3-butadiene (hereafter called polybutadiene), and to said compositions cured with high energy, ionizing radiation. More particularly, this invention relates to a process of curing blends of polyethylene and polybutadiene which comprises treating said blends with high energy, ionizing radiation.

Among the polymeric materials which have evolved in recent years, polyethylene has proved to be one of the most popular. It has found wide usage as an insulating material, as a container material, as a conduit material, etc. Fabrication, molding, extrusion and calendering of polyethylene are readily accomplished by standard methods, thus facilitating its use for many purposes. Despit all this, however, the applications of polyethylene are greatly limited by its lack of form stability, that is, the ability to retain a particular shape at elevated temperares, and by its poor high temperature properties, such as poor high temperature tensile strength, tear strength, cutthrough strength, etc.

One of the methods employed in improving the physical properties of various polymers is the incorporation of fillers therein. Although marked improvement in high temperature physical properties of polyethylene is noted when certain fillers are incorporated therein, the presence of fillers in polyethylene tends to diminish some of the electrical properties of filled polyethylene as compared to the unfilled polymer.

We have now discovered that when blends of polyethylene and polybutadiene are treated with high energy radiation, products having improved physical properties, such as improved tensile strength and elongation, excellent electrical properties, etc. are produced. The significance of this discovery is that cured products having these excellent properties can be produced at room temperature within a very short period of time without using fillers or chemical curing agents. It could not have been predicted that these blends would cure so readily with radiation, since as disclosed in the prior art, polybutadiene could be cured by heat and peroxides only over extended periods of time.

The blend of polyethylene and polybutadiene will also be referred to as blends and these blends cured with high energy, ionizing radiation as cured blends.

In general, the invention can advantageously be carried out by milling polyethylene and polybutadiene on differential rubber rolls (which can advantageously be heated if desired) until a Well-mixed blend is obtained.-

Since it is more difficult to obtain a homogeneous blend at lower temperature, milling of the ingredients is generally carried out at elevated temperatures, such as at about l00l35 C. Thereupon, the blend can be fabricated, molded, extruded or calendered, etc. by suittable methods and the product so treated is cured upon exposure to high energy radiation.

The polyethylene referred to herein is a polymeric material formed by either the high and low pressure polymerization of ethylene. It is described in Patent 2,153,553-Fawcett et al., and in Modern Plastics Encyclopedia, New York, 1949, pp. 268-271. Specific examples of commercially available polyethylene are the polyethylene sold by E. I. du Pont de Nemours & Co., Inc., Wilmington, Delaware, examples of which are Alathons 1, 3, 10, 12, 14, etc., those sold by the Bakelite Company, such as DE-2400, DYNH, etc., and the low pressure Phillips Petroleum Company polymers, such as Marlex 20, 50, etc. An excellent discussion of low pressure polyethylene within the scope of this invention is found in Modern Plastics, vol. 33, #1 (September 1955), commencing on page 85.

1,3 butadiene can enter into a polymer chain by either a 1,2 or 1,4-mode of addition; the 1,2-mode of addition results in the following dangling vinyl structure l ill.

(hereafter called 1,2-polybutadiene) whereas the 1,4- mode of addition results in the following structure:

(hereafter called IA-polybutadiene). Two types of catalysts are generally used to polymerize 1,3-butadiene, namely the free-radical and the alkali metal type catalysts. When 1,3-butadiene is polymerized by free-radical type catalysts, such as peroxides, persulfates, etc. in an aqueous emulsion, a higher proportion of 1,4-polybutadiene results as compared to the product produced by the alkali metal type catalyst where a higher proportion of 1,2-polybutadiene is obtained. Using free-radical catalysts, one obtains polybutadiene having less than 25% 1,2-polybutadiene.

Although blends of polyethylene and both alkali metal polymerized butadiene (also called alkali metal polybutadiene), and free-radical polymerized butadiene (also called emulsion polybutadiene) can be cured with high energy, ionizing radiation to products of improved properties, such as tensile strength, etc., blends of polyethylene and alkali metal polybutadiene can be cured with high energy, ionizing radiation to products of more enhanced properties than the corresponding blends containing emulsion polybutadiene. This may be due to the fact that alkali metal polymerized butadiene which contains larger amounts of dangling vinyl groups (1,2- polybutadiene) is more reactive to the cross-linking influence of high energy, ionizing radiation than is freeradical cured butadiene whose has its residual double bonds buried in the chain of the 1,4-polybutadiene. Thus, in order to obtain the more enhanced properties, such as high tensile strengths, it is necessary to employ polybutadiene containing high percentages of the, 1,2- type, i.e. over 30% and preferably 50-100% 1,2-poly butadiene.

Among the alkali metal type catalysts which have been used to produce polybutadiene having high percentages of 1,2-polybutadiene are alkali metals and compounds containing alkali metals. Thus, metals, such as lithium, sodium, potassium, rubidium, cesium, sodium-potassium alloys, and compounds of these metals, such as phenyl isopropyl potassium, triphenyl methyl sodium, lithium butyl, amyl sodium, and the like compounds have been used to effect such polymerization.

Whereas free-radical catalysts tend to produce larger amounts of 1,4-polybutadiene, catalysts of the alkali sodium at 110 C. contains about 15% of the 1,2-polybutadiene whereas 100% of 1,2 type polymer is produced when 1,3 butadiene is polymerized with sodium at 7 C. Although the ratio of the 1,2- to the 1,4-polybutadiene can be determined by ozonization, probably the more accurate method of determining this ratio is by the use of infra-red spectra. Infra-red curves identifying the different types of polymers are found in Dogadkin et al., Rubber Chemistry and Technology," 24, pp. 591-596 (1951); Hampton, Anal. Chem, 21, pp. 923-926 (1949); and Meyer, Ind. Eng. Chem., 41, pp. 1570- 1577 (1949). An excellent description of polybutadiene polymers is found in Whitby, Synthetic Rubber, pp. 734-757, Wiley and Sons, N.Y. (1954), wherein are described methods of preparing polybutadiene falling within the scope of this invention.

Since molecular weight is related to viscosity, viscosity measurements are a convenient method of expressing the molecular weight. Although polybutadiene gums of a broad intrinsic viscosity range can be employed, we advantageously have employed polybutadiene having an intrinsic viscosity of about 1.0 to 8.0 or higher. Optimum properties are obtained using polybutadiene having an intrinsic viscosity of 3.0 to 6.0.

Inherent viscosity is determined by a viscometer, such as an Ostwald viscometer on a 0.25 percent solution of polybutadiene in benzene. This value is calculated as the natural logarithm of the ratio of flow time of the solution to the how time of the solvent divided by the concentration in grams/100 ml. Intrinsic viscosityl'n] is obtained by extrapolating the inherent viscosity vs. concentration curve of zero concentration. In the drawing there is shown high voltage accelerating apparatus 1 capable of producing a beam of high energy electrons for irradiating polymeric materials in accordance with the invention. High voltage accelerating apparatus 1 may be of the type disclosed in Patent 2,l44,5l8-Westendorp assigned to the same assignee as the present application. In general, this apparatus comprises a resonant system having an open magnetic circuit inductance coil (not shown) which is positioned within a tank 2 and energized by a source of alternating voltage to generate a high voltage across its extremities. At the upper end (not shown) of a sealed off. evacuated, tubular envelope 3 is located a source of electrons which is maintained at the potential of the upper extremity of the inductance coil, whereby a pulse of electrons is accelerated down envelope 3 once during each cycle of the energizing voltage when the upper extremity of the inductance coil is at a negative potential with respect to the lower end. Further details of the construction and operation of high voltage accelerating apparatus 1 may be found in the aforementioned Westendorp patent and in Electronics, vol. 16, pp. 128-133 (1944).

To permit utilization of the high energy electrons accelerated down envelope 3. there is provided an elongated metal tube 4, the upper portion 5 of which is hermetically sealed to tank 2, as illustrated, by any convenient means, such as silver solder. The lower portion 6 of tube 4 is conical in cross-section to allow an increased angular spread of the electron beam. The emergence of hi h energy electrons from tube 4 is facilitated by an end-window 7 which may be hermetically sealed to tube 4 by means of silver solder. End-window 7 should be thin enough to permit electrons of desired energy to pass therethrough but thick enough to withstand the force of atmospheric pressure. Stainless steel of about 0.002 inch thickness has been found satisfactory for use with electron energies above 230,000 electron volts or greater. Beryllium and other materials of low stopping power may also be employed effectively. By forming end-window 7 in arcuate shape as shown, greater strength for resisting the force of atmospheric pressure may be obtained for a given window thickness. De-

sired focussing of the accelerated electrons may be secured by a magnetic-field generating winding energized by a source of direct current 9' through a variable resistor 9.

In producing cured blends according to the invention, a sheet or container thereof is supported in the path of the electrons emerging from end-window 7 as illustrated. The high energy electrons penetrate the polymeric material to a depth dependent upon their energy and effect the above modifications in the properties of the mate rial. Of course, blend 10 can be in the form of a strip material which is passed continuously under end-window 7 at a velocity selected to give the desired irradiation dosage. Other expedients for obtaining the irradiation of these blends in various shapes (e.g., bottles, cups, tubing, filaments, pipes, etc.) will be apparent to those skilled in the art. Uniform treatment of polymeric materials having appreciable thickness can be assured by irradiating them first from one side and then the other or in some cases from both sides simultaneously. In certain instances, it may be desirable to irradiate the polymeric materials in an atmosphere of nitrogen, argon, helium, krypton, or xenon, etc., to prevent the damaging effect of any corona which may be present.

The measure of the amount of irradiation is a Roentgen unit (R) which, as usually defined, is the amount of radiation that produces one electrostatic unit of charge per milliliter of dry air under standard conditions and, as employed herein, refers to the amount of electron radiation measured with an air equivalent ionization chamber at the position of the upper surfaces of the polymeric materials. A million Roentgen units (1x10 R.) is designated by the term MR.

The total irradiation dose to which the polymer is subjected will depend on the properties desired in the crosslinked product. Thus, a total dose of from 10 to 10 R. or higher can be employed, but preferably 10 to 10 R. Irradiation can be carried out below room, at room, or at elevated temperatures.

It will be readily realized that other forms of electron accelerating apparatus may be employed instead of high voltage apparatus 1; for example, linear accelerators of the type described by J. C. Slater in the Reviews of Modern Physics," vol. 20, No. 3, pp. 473-518 (July 1948), may be utilized. To decrease wasteful energy absorption between the point of exit of electrons from the accelerating apparatus and the polymeric materials, a vacuum chamber having thin entrance and exit windows may be inserted in the space.

In general, the energy of the irradiation preferably employed in the practice of our invention may range from about 50,000 to 20 million electron volts or higher depending upon materials. Although high energy electron irradiation is preferred, since it produces a large amount of easily controllable high energy, ionizing radiation within a short period of time without rendering the product radioactive, many other sources of high energy, ionizing radiation can also be used in my invention. Examples of such radition sources are gamma rays, such as can be obtained from Co, burnt uranium slugs, fission by-products, such as waste solution, separated isotopes, such as Cs gaseous fission products liberated from atomic reactions, etc.; other electron sources, such as the betatron, ets.; fast or slow neutrons or the mixed neutron and gamma radiation, such as is present in certain atomic reactors; X-rays; and other miscellaneous sources, such as protons, deuterons, tit-particles, fission fragments, such as are available from modern cyclotrons, etc.

In order that those skilled in the art may better understand how the present invention may be practiced, the following examples are given by way of illustration and not by way of limitation. All parts are by weight.

EXAMPLE 1 A rubbery polymer was prepared from LS-butadiene able to vary tensile strength and elongation. Thus, blends containing the following ratios, 95/5, 60/40, 50/50 can t be irradiated with doses of 20X 10 R. to 1X 10 R. to produce room temperature tensile strengths of 2000-3000 toluene. Thereafter, 25 g. of 1,3-butadiene was charged 5 psi. and 150 C. tensile strengths of 500-900 p.s.i. Of as a liquid. A small amount of the butadiene was 91- course, these values will vary with total irradiation doses lowed to evaporate to displace any air remaining in the which can be varied with the thickness of the sample. Albottle. The bottles were capped, and rotated at 30 C. though a wide range of dosages can be employed with for a period of 48 hours. The residual catalyst was dethese compositions, we advantageously employ R. to activated by adding ml. of a 10% solution of absolute 10 10 R. but preferably 10 R. to 10 R. Furthermore, dealcohol in benzene. The rubber was recovered by precipipending on desired properties, the ratio of polyethylene tation from a benzene solution by addition of ethyl alcohol to polybutadiene can be widely varied, for example from until polymer no longer precipitated. To this precipitated about 95/5 to 5/95. product was added 0.1% of phenyl-fi-napthylamine as an In addition to improved tensile strength, the composiantioxidant based on polymer. This unwashed polymer 15 tions of this invention possess excellent electrical prop had an intrinsic viscosity of 6.0 when measured in benzene erties, such as power factor, dielectric constant, loss facsolution. By infrared analysis, this product contained at tor, alternating current resistivity, direct current resistivity, least 60% of 1,2-polybutadiene. etc. These compositions are also less susceptible to EXAMPLE 2 humidity from an electrical viewpoint as compared to the correspondmg morgamc filler-containing blends. Emulsion polybutadiene Was P p y fiddhlg Since the products of this invention have greater hot parts Of 1,3-butadiene to a solution Of 1.25 strength than polyethylene compositions previously de. Parts of p flakes y P of Ph scribed, they can be used in applications where polyethyl- Shlm Persulfate and P Of dodeeyl meleaptah 1h 45 ene itself has failed due to high temperature form insta- Parts of Watef- The reaction Vessel was capped and 25 bility. Thus, these products can be used in hot strength tilted eontihllehsly at for 43 hours, then eboledfo films or tapes for electrical insulations, for electrical parts, the temperature of ice-water, and the contents added with f example, Spark l caps, fo household utensils which stirring to e eoheehh'ated aqueous Solution of are used at elevated temperatures, for molded industrial Sodium ehlerlde- Thefellpbn 200 of a 2% 2 4 parts which are subjected to high temperatures, for ex- SOhlIien was added the y- After the Prmhlet was ample, jet fuel cartridges, and the like, for industrial freed of acid and 531th y Water washhlge, Water was f laminates, for conduits and containers for hot liquids, etc. moved y Washing Wlth alcohol and Plaelhg the leshlhhg as well as for other uses which will appear to those skilled product in a desiccator for 48 hours to remove the residual in the an alcohol- All antioxidant, p y -fi p y 01% Although the presence of fillers tends to .diminish the based on Pb y was then milled E ep electrical properties, their presence is not precluded for In addition to the method described In E p 2, certain applications; for example, conducting carbon emulslofl pelymeflled pblybhiadlehe can be P p by blacks and metallic particles can be incorporated in these t e methods k110 WIl t0 f as 9} example, those blends for strong but flexible heating pads and tapes. For methods dlselosed 1h y, Syhthetle Rubber, John other applications where electrical properties are of secwlley & Sons PP- Q ondary importance, it may be desirable to add other fillers, A hlend of 20 Parts of sodlum polybutadlene (Pre' such as finely divided silica aerogels, xerogels, fumed pared in the manner of Example 1) and 80 parts of polysilicas, such as aerosils, silicas rendered hydrophobic by ethylene (Alathon 10) w Prepared by mlumg these surface treatments with alcohols in the manner of US. gred ients together on difierential rubber rolls heated to Patent 2,657,149 ner and malkylsflanes in the manner 130 C. A corresponding blend of 20 parts of emulsion f B h t l S l N 531 829 fil d polybutadiene (prepared in the manner of Example 2) and 0 e e a Ion ena e 80 parts of polyethylene (Alathon 10) was similarly pre- August assigned to the same asslgne? as h pared. A 29 mil film of each of these compositions was Present R abandoned Calclum S111 prepared and irradiated with high energy electrons derived Gates alummas vanous kmds of f f black and P h from a 800 kilovolt peak (kvp.) resonance transformer to fillers can also be used; In hq other f f a total dose of 20 l0R. and the properties of the cured agents, Such as y pigments, stablllzers, plastlclzers, product were compared with the corresponding unirradiantiOXidantS, can also be added without departing ated blend. These results are shown in Table I. from the scope of the invention.

Table I Non-irradiated Irradiated Ex. Composition Tensile Percent Temper- Tensile Percent Temper- Stren th, Elongaature, Strength, Elongaature,

p.s. tlon 0. p.s.i. tlon 0.

3 52 5222:assessment..- mats s. as 388 as 4 tfi ggt%%l;hi% 2i;ir was m aa a w s 1 Too soft to measure.

From this table it is evident that the blend of sodium polybutadiene and polyethylene is more readily cured by high energy, ionizing irradiation than the corresponding blend containing emulsion polybutadiene. By increasing the dose, a more highly cross-linked material is obtained.

By varying the ratio of polyethylene (Alathon 10) to sodium polymerized polybutadiene (Example 1), we are polyethylene based on the total weight of (1) and (2) said blend being cured by high energy, ionizing radiation having energy equivalent to at least 5x10 electron volts to a dose of at least 10 R.

2. A process of curing polymers which consist essentially of blends of (1) polyethylene and (2) polymerized 1,3-butadiene containing at least 30% 1,2-polybutacliene said polyethylene being present in the range of 5 to 95% by weight of the total weight of (1) and (2) which comprises treating said blends with high energy, ionizing radiation having energy equivalent to at 10 least 5x10 electron volts to a dose of at least 10 R. 3. The composition of claim 1 in which the polymerized 1,3-butadiene comprises at least 50% 1,2-polybutadiene.

4. The process of claim 2 in which the polymerized, 1,3-

butadiene comprises at least 50% 1,2-polybutadiene.

5. The composition of claim 1 wherein electrons are the source of high energy, ionizing radiation.

6. The composition of claim 5 in which the polymerized 1,3-butadiene comprises at least 50% 1,2-polybutadiene.

7. The process of claim 2 in which electrons are used as a source of high energy, ionizing radiation.

8. The process of claim 7 in which the polymerized 1,3- butadiene comprises at least 50% 1,2-polybutadiene.

References Cited in the file of this patent UNITED STATES PATENTS 2,369,471 Latham Feb. 13, 1945 FOREIGN PATENTS 430,775 Canada Oct. 23, 1945 OTHER REFERENCES Whitby: Synthetic Rubber, pp. 295, 734, pub. 1954.

Sisman et al.: 0RNL-028, pp. 9 to 26, 78 to 92, June 29, 1951. (Copy from Technical Information Service, Oak Ridge, Tenn.)

Bopp et al.: ORNL-1373, pp. 1 to 18, 25, 26, 32, 33, 36, 37 and 52 to 71, July 23, 1953. (Copy from Technical Information Service, Oak Ridge, Tenn.)

Lawton et al.: Nature, vol. 172, pp. 76, 77 (July 11, 1953).

Lapp et al.: Nuclear Radiation Physics, pp. 433 to 436, 1948, Prentice-Hill, Inc., N.Y.C.

"BNL 367, cover ii, pp. 27 and 28, February 1956. 

2. A PROCESS OF CURING POLYMERS WHICH CONSIST ESSENTIALLY OF BLENDS OF (1) POLYETHYLENE AND (2) POLYMERZED 1,3-BUTADIENE CONTAINING AT LEAST 30% 1,2-POLYBUTADIENE SAID POLYETHYLENE BEING PRESENT IN THE RANGE OF 5 TO 95% BY WEIGHT OF THE TOTAL WEIGHT OF (1) AND (2) WHICH COMPRISES TREATING SAID BLENDS WITH HIGH ENERGY, IONIZING RADIATION HAVING ENERGY EQUIVALENT TO AT LEAST 5X10**4 ELECTRON VOLTS TO A DOSE OF AT LEAST 10**6 R. 